Gary, I think that the John Parthemore, witness to the promisory note between John Argenbright and his father, Jacob, dated 19 Feb 1802, probably in Augusta or Rockingham County, Virginia, was most likely John B. Parthemore, 1764-1840.The reasons:
Walter L. Jebens and Dr. Udo P. Krauthausen's book, "Partenheimer, Parthemer, Parthemore Family History 1545-2002," (pages 149-150), shows that John B. Parthemore, the son of Johann "John" Philip Parthemore and Anna Eva Hypsher of Middletown, Lancaster/now Dauphin County, PA, was born 22 Jan 1764 in Derry Township, Lancaster/Dauphin Co., PA; died 11 Sept 1840 in Union Co., Ohio.(John Philip Parthemore, 1736-1797, was your ancestor's, Susanna Argenbright's brother, so Susanna was John B. Parthemore's aunt.)
John B. Parthemore and his wife, Anna Catharine Shell, moved to the Shenandoah Valley of Virginia about 1798.Jebens and Krauthausen show that of their 11 children, 7 of their children were born in Augusta County, Virgina.By 1812 the family moved to Ross County, Ohio -- where John's lst cousin, Maria Elizabeth (Parthemore) Beyerly/Byerly, lived.You have shown that his first cousin, John Argenbright, who lived in Augusta County, VA, also moved to Ross County, Ohio.John B. Parthemore, his wife, and their children moved to Union County, Ohio in 1814.
Since Susanna (Parthemore) Argenbright's two brothers, John Philip Parthemore and Johannes (John) Parthemore, both remained in Lancaster/Dauphin County, PA, it seems reasonable that it was probably Susanna's nephew, John B. Parthemore, who was the witness.